Although the heart has its own natural pacemaker that sets its rhythm, the term “pacemaker” most commonly refers to an artificial electronic device that is implanted in the chest to generate and regulate the heart’s rhythm. Generally, pacemakers send electrical impulses to one or more chambers of the heart. These signals make the heart contract in a more regular rhythm than the chamber would otherwise. Pacemakers are most often used to treat cardiac conditions that involve bradycardia (an abnormally slow heartbeat). Symptoms of bradycardia include dizziness, light-headedness, fainting, fatigue and poor tolerance for exercise. Following a permanent pacemaker insertion, most patients with these conditions report significant improvements in their quality of life.

There are several kinds of permanent pacemakers, each designed to meet a different need. They include:

• Demand pacemakers, which monitor the heart rate and discharge electricity only when the heart rate falls below a programmed minimum or misses a beat.
  
  
• Fixed-rate pacemakers, which discharge a steady stream of electrical impulses, regardless of the underlying heart rate.
  
  
• Rate-responsive pacemakers, which monitor various physical changes in the body (e.g., respiration, physical activity) and change the rate of discharge accordingly.

In addition, pacemakers differ in which areas of the heart they stimulate. So-called “single lead” pacemakers stimulate only the upper chambers of the heart (atria). Newer “double lead” pacemakers stimulate both the upper and lower chambers (ventricles) of the heart to more accurately mimic a natural sequence of heart rhythm. Pacemakers are also designed that simultaneously stimulate both ventricles of the heart, sometimes in addition to pacing the right atrium (e.g., a triple-lead pacemaker). The device is known as a biventricular pacemaker, and the therapy is called cardiac resynchronization therapy. Finally, pacemakers can be built with a special device that shocks the heart back into a normal rhythm if it stops or experiences a serious rhythm abnormality. These devices are called implantable cardioverter defibrillators (ICDs). The choice of which pacemaker to use depends on the patient, their lifestyle and the condition. Underlying heart conditions that may warrant a permanent pacemaker include:

• Sinus node dysfunction. Also called sick sinus syndrome, this occurs when the body’s natural pacemaker sends out electrical impulses too slowly. It is the leading cause of permanent pacemaker implantation. A pacemaker will almost always be recommended if the heart rate drops below 40 beats per minute or if symptoms are clearly related to the bradycardia. They may be recommended if the heart rate is greater than 40 beats per minute and there are symptoms that are suggestive of bradycardia. In general, patients without symptoms are not candidates for a pacemaker, even if they have an unusually slow heart rate. 
  
  
• Heart block (also known as atrioventricular block). Heart block occurs when there is a delayed or complete lack of communication between the upper and lower chambers of the heart. It is the second most common reason for pacemaker implantation. Heart block is commonly the result of degeneration of the heart’s normal conduction system, although it may also be caused by a number of diseases, including muscular dystrophy. The term conduction system refers to the pathway of specialized fibers that carry electrical impulses through the heart in a predictable fashion. An inability to conduct electrical impulses between the upper and lower chambers of the heart results in a slow heartbeat. This is one reason why people are more likely to need pacemakers as they age. In fact, most of the nearly 200,000 pacemakers implanted annually in the United States are in patients ages 65 or over. However, complete heart block may also be present a birth (e.g., a congenital heart defect).  
  
  
• Carotid sinus hypersensitivity. A hyperactive reflex causing the heart rate to drop sharply when pressure is applied to the neck.
  
  
• Arrhythmias arising from ablation. Ablation is a minimally invasive procedure sometimes used to terminate abnormal electrical discharges in the heart. It can result in bradycardia. 
  
  
• Following a heart attack with partial or complete heart block that is not resolved.
  
  
• An abnormally slow heart rate due to long-term drug therapy.
  
  
• Heart failure that involves the heart’s conduction system. Among these patients, cardiac resynchronization therapy is typically recommended for patients who have symptoms and are class III or class IV according to the New York Heart Association classification system.
  
  
• Heart transplant patients may require permanent pacing if they experience bradycardia after their surgery.

In general, a pacemaker consists of three parts:

• Generator
• Leads
• Electrodes

The generator is a small box, usually about 2 inches wide and approximately 3 ounces in weight. Some generators are even smaller, measuring 1 inch in diameter and weighing about half an ounce. They are battery-powered, and most use lithium batteries that last for 5 to 10 years. When the battery runs out, the entire generator is replaced. The generator is responsible for generating the electric impulses that correct the slow heartbeat.

Attached to the generator are one or more leads, or wires, generally made of platinum with an insulating coating of either silicone or polyurethane. The leads carry the electrical impulses from the generator.

At the tip of each lead is a tiny device called an electrode that delivers the necessary electrical impulses to the heart. Thus, the electric impulses are created by the generator, carried by the leads and delivered by the electrodes to the heart.